You'll have to instantiate the signal in a file called "signals.py" in your app's folder:

object_viewed=object()

And then, in any file in which you use that signal (typically, models.py and views.py), you'll need to import your signals file:

from project.app import signals

Example

The following is taken from ​Zyons, the only place I've seen signals used so far. Unless I'm completely incorrect, the following code has the dispatcher call "increment_tag_summary" before a TagUserObject object is saved, and "decrement_tag_summary" after a TagUserObject object is deleted.